I'm sorry you're having to learn by trial by fire. I had a similar situation, 3 new 9 week old birds brought in when I first got into chicken keeping. There's not enough talk about the risks in bringing in new birds! (And i got the new birds from the same dealer I got the day old chicks from! You never know.). Those birds infected my flock with coccidiosis even after a 30 day quarantine. Since then my rule has been: no new adult birds join the flock. Ever. Don't beat yourself up: this is how we chicken keepers learn.
That being said, if that bird was in my flock, it would be an immediate culling. There is no curing a respiratory disease; sick birds are carriers for life. What's worse, a chicken that falls ill like this makes the rest of your flock that much more vunerable to illness. I'd also cull the birds you got from your friend, immediately. If you're lucky, some of your original flock may escape getting sick. If you decide to just treat the illness, keep in mind that all future birds you get are more likely to get sick, and you'll be compounding the problem.
My two favorite books at the moment are: "The Small Scale Poultry Flock" by Harvey Ussery and "Chicken Health for Dummies" by Julie Gauthier. They may offer more inisght, too. Good luck on whatever you decide.
